Mixer circuit



" Aug. 15, 1950 I R. B. DOME 2,519,030.

MIXER CIRCUIT Filed 001'.. 29, 1946 illfillilll Invent. or": Robert B.Dome,

t-Us Att ornga.

- um-es sms Patented Aug. 15,1950

I General Electric Co ;New York Robert B. Dome, Bridgeport, Conn.,assignor to mpamf, a corporation of l ienatio October 29, 1 94 serialNo. 7os,447f

invention relates to electronic tube circuits for arithmeticallycombining two distinct high frequency signals, and has for itsprimaryobject to provide a new andimproved electronic tube circuit formixing two signalsin which the intensity of the signals may be variedindependently. V

In high frequency apparatus'of the type empioyed for locating objects inspace, it is customary to employ a cathode ray' tube, known as a planposition indicator, to which are supplied signals reflectedfrom anobject, as well as a periodic signal of relatively highfrequency whichis employed to indicate" electronically on the oathode ray tube theazimuth angle of an antenna which receives the reflected signals. It iscustomary to combine therefiected and high frequency signals prior toapplication to a controlelectrode of the cathode ray tube. One circuitused for combining two such signals comprises a pair of triodes having acommon cathode resis'tance, the reflected signal as well'a's theperiodically repeated signal'b'eing supplied to the two controlelectrodes of the triodes. One of the difficulties encountered insuch acircuit, however, is that, when the i'ntensity'of one of thesignalssupplied to acontrolelectrode is varied in order to control theindication on the'c'athode ray tube, the intensity of theother signal isadversely afiected, the-two signals being interdependent because oftheinterelectrode capaci tance inherent in the electronic tubesemployed. Accordingly, it is another object of my invention to provide anew and improved mixer circuit having a pair of signal input circuitsand a single output-circuit in which the amplitude of one input signalmay be varied without affecting the tion taken in connection with 'theaccompanying drawing in which the single. figure represents anembodimentof my'invent'ion. i Referring to the drawi I have indicatedtherein an electron discharge device I of the double triode type. One ofthe 'triodes comprises" a cathode. 2,'a control electrode Bland an anodeAl; .The other triode. sectioncomprises a cathode 3 Claims. (oi. 250-2r)5, a control electrode 6, and an anode]. Th e cathodes 2, 5 areconnected together externally of the tube and are connected to groundthrough a common cathode resistance 8; Control 5 electrode 3 isconnected through a variable con- 10 able resistance or gain controlpotentiometer 12::

of which'the contact 9' forms the variable ta'pl; The source of signalsI'll may, for example, be-

i very short pulses, such as are used in radio 10-? cating and detectingequipment for measuring:

15 the range of a reflecting object. The signals l3 are indicated asvery short pulses which occur: at regularly spaced intervals of timeequivalent *to a determinable interval in the range The anode 40f theleft-'h-and trio'de is con nected through a resistance [4 to thepositive;

terminal of a source of anode potential indi'catedlas a battery 5, thenegative'terminal of which is{ connected to ground, A second source Itof high frequency signals is coupled to 't'hecontrol'grid 6* through acoupling'capacitor l1 and a gain con-f trol potentiometer l8 having a'v'ariabl t'apflQ' connected to control electrode 6. l The signals'ltmay be, for example; sinusoidal oscillationsofa" relatively highfrequency and'may be employedfor electronically producing azimuthalangle" marks on a cathode y tube used as a3p1an d sition indicator in aradio detection and ranging system. The anode"! of the right-handtriode'i's connected to the positive terminal of batte 15 through aplate coupling resistor '20. The-com bined signals in the outputs of thetwo triode sec-" tions are supplied througha coupling capacitor 532-! toa pair of output terminals 22, 2 3;

In the operation of the mixer circuit described;

40 the left-hand, triode functions as a cathode follower so that signalsfromthe source l0; appear"- across cathode resistor '8 and arecontrollable in magnitude by varying the position of tap 9 ongaincontrol potentiometer l2. Since the oath-* ode resistor '8 is commonto the input circuits of":

bothitriodes, variations in voltage across resistor- '8 are included inthe input circuit of the righthand triode and, hence, act to control-theplate current-of the right-hand'triode so'that signals from:the source.It! appear across the resistor 20 and at; the output terminal 22.Theright-hand triode functions also as a conventional amplifier 51-1; toamplify signals from the source It inth'usualf.

manner; 1

55 Although the signals from both the'sour'ces l 0,

[6 appear across theoutput terminals 22; :23 and either, may: becontrolledin amplitude :by varlationzof respective taps 9 and fl 9;there is present an undesirable interlocking of the gain control action.For example, if the gain control for the right-hand triode is varied,the amplitude of signals coming from source ID is likewise varied asmeasured-between the output terminals. This is by virtueuoi the factthat, because of the interelectrode capacitance between cathode andcontrol electrode 6, the potential of the control elec trode 6 does notstay at zero but varies with the variation of potential of cathode 2,that is, if the potential of cathode 2, and consequently cathode 5,is'decreased by decreasing the intensity of the signals from source Insupplied to control electrode 3, the potential of grid 6 likewisedecreases. The result is that the efiective grid-to-cathode potential ofthe right-hand triode is decreased so that the intensity of the-signalsof the frequency ot the source 16 across the terminals 22, 23 isdecreased,

In. order to eliminate such undesirable inter dependency of gain.control action of the two triodes'and to permit, so far as possible, thevarying ofthe intensity of the two signal sources independently, Iprovide means for neutralizing the effects of the undesired. capacitycoupling between the electrodes of therigh-t-hand triode of tube Thismeans comprises a capacitance 24 whichis; connected between anode 4 andcontrol electrode-6. Insoiar as normal amplification. is concerned, Ythe circuit with the addition of capacitance .24: operates as explainedabove. In addition, however, by suitably choosing the valuesofresistorI14 and capacitance 24, any action of cathode 5 tending to charge thecontrol electrode 6 in one polarity is exactly-counterbalanced byanoppcsite .pola-rity charging eifect through ca-v pacitance 2 4. As aresult, the net charge on the control electrode 6 is unvaried by anyvariation or th intensity ofsignals applied to control electr e 3 sothat the ratio of the voltages of the twosou-rces across the terminals22, 23 is independent of the impedance between control electrade 6 andground. Hence, changing the resistance between tap l9 and ground doesnot atfeet-the amplitude of the signals l3 which appear across the.terminals 22, 23; In this circuit, the resistor in the anode circuit ofthe left-hand triodeiunctions toprovide a source of potential varyingwith the variations of the signals of source I 0 and. with oppositepolarity to neutralize the'effect of the above described interelectrodecapacitance. In constructing a circuit of this type, the values of theimpedance elements i4, 24 very inversely, that is, for a larger value ofresistance [4, a smaller value of capacitance 24-,

,whiie .inthe drawing and foregoing descrip tion I have described thetwo triodes as being enclosedin a single envelope, fit is of courseapparent that separate envelopes'may be provided for the twoftriodesyand the grid-to-cathode capac'itance of one of the triodesneutralized in the manner described and illustrated.

j ;While Ihaveshown a particular embodimentof my inventiomit will ofcourse be understood that Irdoanotwish tobe limited thereto sincedifferent modifications may be made both in the circuit arrangements andthe instrumentalities employed and I contemplate bythe appended claimsto cover all such modifications as fall within the true spirit and scopeof my invention.

What I. claim as newand desire to secure by Letters Patent of the UnitedStates is:

1. In combination, a mixer comprising a pair of electron dischargedevices, each comprising an anode, a cathode, and a control electrode,said cathodes being connected through a common resistance to a point offixed potential, a pair of sources of signals of variable intensityconnected respectively between said point and respective ones of saidcontrol electrodes, whereby undesired capacitance between one of saidcontrol electrodes and its associated cathode causes-a potential of saidone electrode to vary undesirably with variation. in the intensity ofsignals supplied to the other of said control electrodes, and meanscomprising a capacitance connected between. said one electrode and theanode associated with. said other electrode for neutralizing saidundesired capacitance.

2. In combination, .amixer comprising a pair of electron dischargedevices, each comprising an anode; a cathode, and a control. electrode,a

;trodes and its associated cathode causing the potential of said oneelectrode to vary undesirably with variations in. intensity of signalssupplied to the other of said control electrodes, means for producingsignals varying oppositely to H the signals supplied to said. otherelectrode, and

means utilizing said oppositely varying signals for neutralizing theeffect of said undesired capacitance.

3,,In combination, amixer comprising a pair of electron discharge:devices, each. comprising an anode, acathode, and a control electrode, asource of potential having positive and negative terminals, saidcathodes being connected through a common resistance to said negativeterminal,

said anodes being connected through respective resistances to saidpositive terminal, a pair of sources of signalsof'variable intensityconnected respectively between said negative terminal and respectiveones of said control electrodes, un-

' desired capacity coupling between one of said control electrodes andits associated cathode causing-the potential of said one electrode tovary undesirably with variation in the signals supplied tothe other ofsaid control electrodes.

and meanscomprisin-g a capacitance connected between said one electrodeand the anode associated with the'other electrode for neutralizing saidundesired capacitance,

ROBERT B.

REFERENCES CIT-ED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Date Number Name 2,265,996 Blumlein'n. Dec. 16,1941 2,441,954 Chattcrjea May 25,. 1948

